Magnetic small-angle neutron scattering (SANS) is the method of choice for studying spin structures on a mesoscopic length scale of typically 1\(-\)\(100 \, \mathrm {nm}\)  and inside the volume of magnetic materials [1]}, [2]}. A growing number of experimental investigations on magnetic nanoparticles, in particular using polarized neutrons, unanimously suggest that the encountered spin configurations are highly complex and exhibit a variety of nonuniform, canted, or core-shell-type textures (see, e.g., Refs. [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]} and references therein). However, a problem arises since the prototypical magnetic SANS data analysis is largely based on structural form-factor-type models for the cross section. These are borrowed from nuclear SANS and do not properly account for the existing spin inhomogeneity inside magnetic nanoparticles. On the other hand, analytical as well numerical computations of the magnetic SANS cross section [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}, [24]}, [25]}, [26]}, [27]} strongly suggest that for the analysis of experimental magnetic SANS data the spatial nanometer scale variation of the orientation and magnitude of the magnetization vector field must be taken into account; and that macrospin-based models—assuming a uniform magnetization—are not adequate.
